The metal binding sites of a large number of metalloproteins contain imidazole rings from histidine residues of the protein. Examination of the structural data available for these proteins suggest that the ability of these metal-bound imidazole rings to hydrogen bond through the pyrrole nitrogen or to be deprotonated to form a bridge between two metal ions is an important aspect of their chemistry. We propose to investigate the chemistry of the N-H functionality of metal-bound imidazole in model complexes, especially of metalloporphyrins. Our plan is to prepare and characterize new complexes of imidazolate, both mononuclear and binuclear. Furthermore, we will examine the hydrogen bonding properties of metal-bound imidazole by correlation of a number of spectroscopic and redox potential measurements. Using published structural data, we will also investigate the hydrogen bonding of axial histidines in hemoporteins such as hemoglobin where we believe such bonding may be involved in the mechanism of cooperative binding of oxygen. We will also continue our investigation of the role of the imidazolate bridge in Cu-Zn superoxide dismutase.